Conventionally, a latent image formed electrically or magnetically in an apparatus such as electrophotographic image forming apparatus is visualized by a toner for electrophotography (this may also be referred to simply as “toner”). For example, in electrophotography, an electrostatic image (latent image) is formed on a photoconductor, then the latent image is developed with a toner, and a toner image is formed. The toner image is transferred on a transfer medium such as paper and then fixed on the transfer medium such as paper. In a fixing step for fixing the toner image on transfer paper, a heat fixing method such as heat roller fixing method and heat belt fixing method is generally used widely for its energy efficiency.
In recent years, market increasingly demands a high-speed image forming apparatus and saving of energy, and a toner which has excellent low-temperature fixing property and is able to provide a high-quality image is demanded. To obtain low-temperature fixing property of a toner, it is necessary for a binder resin of the toner to have a reduced softening temperature. However, when the binder resin has a low softening temperature, so-called offset that a part of a toner image adheres on a surface of a fixing member during fixing and transfers on copy paper (hereinafter, also referred to as hot-offset) is likely to occur. Also, heat-resistant storage stability of the toner decreases, and so-called blocking that toner particles fuse with one another under a high-temperature environment occurs. In addition, there are problems of contamination that a toner fuses to an inside of a developing device or a carrier and a problem of toner filming on a surface of a photoconductor.
As a technique to solve these problems, it has been known to use a crystalline resin as a binder resin of a toner. That is, the crystalline resin may soften rapidly at a melting point of the resin, and the toner may have a reduced softening temperature close to the melting point while ensuring heat-resistant storage stability at a temperature below the melting point. Thus, it is possible to support both low-temperature fixing property and heat-resistant storage stability.
As a toner using a crystalline resin, toners using a crystalline resin as a binder resin that crystalline polyester is elongated by diisocyanate have been proposed (see PTL1 and PTL2). These toners have excellent low-temperature fixing property but have insufficient heat-resistant storage stability, and they do not reach the quality required in recent years.
Also, a toner using a crystalline resin having a crosslinking structure by an unsaturated bond containing a sulfonic acid group has been proposed (see PTL3). This toner has improved heat-resistant storage stability compared to the prior art so far. Also, a technology of resin particles having excellent low-temperature fixing property and heat-resistant storage stability by defining a ratio of a softening temperature to a heat of fusion peak temperature and viscoelastic properties is disclosed (see PTL4).